diff --git a/csv2bufr/__init__.py b/csv2bufr/__init__.py
index 249eb38..c0d6c0a 100644
--- a/csv2bufr/__init__.py
+++ b/csv2bufr/__init__.py
@@ -45,7 +45,10 @@
 # some 'constants'
 SUCCESS = True
 NUMBERS = (float, int, complex)
-MISSING = ("NA", "NaN", "NAN", "None", "", None)
+MISSING = ["NA", "NaN", "NAN", "None", "", None]
+
+if os.environ.get("CSV2BUFR_MISSING_VALUE") is not None:
+    MISSING.append(os.environ.get("CSV2BUFR_MISSING_VALUE"))
 
 if 'CSV2BUFR_NULLIFY_INVALID' in os.environ:
     NULLIFY_INVALID = os.environ['CSV2BUFR_NULLIFY_INVALID']
@@ -58,7 +61,7 @@
 
 LOGGER = logging.getLogger(__name__)
 
-BUFR_TABLE_VERSION = 42  # default BUFR table version
+BUFR_TABLE_VERSION = os.environ.get("BUFR_TABLE_VERSION", 41)  # noqa default BUFR table version
 # list of BUFR attributes
 ATTRIBUTES = ['code', 'units', 'scale', 'reference', 'width']
 # list of ecCodes keys for BUFR headers
diff --git a/docs/source/installation.rst b/docs/source/installation.rst
index 7b9f8a2..3a129b5 100644
--- a/docs/source/installation.rst
+++ b/docs/source/installation.rst
@@ -85,9 +85,12 @@ Three environment variables are defined and can be used to set the originating c
 sub centre of the generated BUFR files and the system search path used to find BUFR mapping templates(see
 :ref:`BUFR template page <mapping>`)
 
-- ``BUFR_ORIGINATING_CENTRE``: Specifies the originating centre of the BUFR data, Common Code Table C-11
-- ``BUFR_ORIGINATING_SUBCENTRE``: Specifies the originating sub centre of the BUFR data, Common Code Table C-12
-- ``CSV2BUFR_TEMPLATES``: Path to search for BUFR templates
+- ``BUFR_ORIGINATING_CENTRE``: Specifies the originating centre of the BUFR data, Common Code Table C-11, defaults to BUFR missing value.
+- ``BUFR_ORIGINATING_SUBCENTRE``: Specifies the originating sub centre of the BUFR data, Common Code Table C-12, defaults to BUFR missing value.
+- ``BUFR_TABLE_VERSION``: Default BUFR table version number to use if not specified in mapping template, defaults to 41.
+- ``CSV2BUFR_MISSING_VALUE``: Value used to indicate missing value in csv input file if not in ("NA", "NaN", "NAN", "None", "", None).
+- ``CSV2BUFR_NULLIFY_INVALID``: True|False. If True invalid values are set to missing (with warning), otherwise error raised. Defaults to True if not set.
+- ``CSV2BUFR_TEMPLATES``: Path to search for BUFR templates, defaults to current directory ("./"). Paths are searched in order of ``CSV2BUFR_TEMPLATES`` and then ``/opt/csv2bufr/templates`` (if exists).
 
 Note: the ``BUFR_ORIGINATING_CENTRE`` and ``BUFR_ORIGINATING_SUBCENTRE`` are only used if missing from the
-specified :ref:`mapping file <mapping>`).
\ No newline at end of file
+specified :ref:`mapping file <mapping>`.
\ No newline at end of file